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15-05-2012 | Pharmacology | Article

Watching needle prick during vaccination increases pain felt


Free abstract

MedWire News: Researchers have found that previous experience and situational expectations may shape the impact that viewing a needle prick may have on pain perception.

"Throughout our lives, we repeatedly experience that needles cause pain when pricking our skin, but situational expectations, like information given by the clinician prior to an injection, may also influence how viewing needle pricks affects pain," said lead author Marion Höfle (University Medical Center Hamburg-Eppendorf, Germany) in a press statement.

Her team's study included 25 participants, 13 of whom were women, with a mean age of 26.8 years and normal or corrected-to-normal vision, no history of neurologic or psychiatric illness, and no acute pain.

The participants were presented with video clips of a hand perceived as their own being either pricked by a needle or touched by a Q-tip, while concurrent painful or nonpainful electric stimuli were applied to the tip of their left index finger. A clip of a hand alone was used as the control condition.

Following presentation of the last frame of each video clip, each participant rated the intensity and unpleasantness of the electric stimulus on a 2D visual analog scale. On the vertical intensity axis a score of 0 corresponded to no sensation and 100 to most intense pain, and on the horizontal unpleasantness axis, a score of 0 corresponded to "not unpleasant at all" and 100 to "extremely unpleasant."

Pupil dilation response (PDR) was also assessed using an infrared eyetracking system.

To study the effect of situational expectations on pain perception, the contingencies differed between video clips (needle vs Q-tip) and strength of electric stimulation (painful vs nonpainful). Participants were explicitly informed about the contingency of the stimuli by pictorial and verbal instructions.

The participants reported that when they viewed a needle pricking a hand, their pain was more intense and unpleasant than when they saw a hand alone. In addition, the unpleasantness when viewing needle pricks was greater than when they viewed Q-tip touches.

These results were corroborated by the PDR findings, which were significantly higher when patients viewed a needle prick than when they saw a Q-tip touch. This demonstrates that previous experience with viewing needle pricks acts primarily on perceived unpleasantness, the authors say in Pain.

Situational expectations also influenced pain intensity. Indeed, when patients were informed that a certain clip was associated with pain, they were significantly more likely to report a higher pain intensity than when shown clips that were not associated with pain. The authors say that this suggests situational expectations about forthcoming pain bias its perceived intensity.

Höfle recommended: "Clinicians may be advised to provide information that reduces a patient's expectation about the strength of forthcoming pain prior to an injection."

"Because viewing a needle prick leads to enhanced pain perception as well as to enhanced autonomic nervous system activity, we've provided empirical evidence in favor of the common advice not to look at the needle prick when receiving an injection."

By Piriya Mahendra

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